Tips from the Architect: The Role of Windows in Home Insulation

December 22, 2015

Catalano Architects recently attended the 2015 Architecture Boston Expo (ABX) at the Boston Convention and Exhibition Center.  ABX offers a wide range of lectures on various architectural, design, and building topics and also has an extensive showroom highlighting vendors and a wide variety of upcoming and highly used products. Catalano attended a lecture by Steve Easley entitled “Understanding High Performance Window Technologies”. Below we share a few helpful highlights.

Efficient home insulation is important, especially for those of us living in Northeast climates. In America, the standard window-glazing ratio is 15% of the house. This means that widows have a huge impact on the thermal resistance, or R-Value, of the house. Thermal resistance is the measurement of an insulating material’s resistance to conductive heat flow. The higher the R-Value, the more effective the material is as an insulator.


Window Energy Performance Ratings  

Window manufacturers base their window ratings on four factors. The ratings include the entire window unit (frame, glass, etc.).

U-Factor: This is the measure of how well the window unit prevents heat transfer. The U-Factor is equal to the inverse of the R-Value. The lower the number, the better the insulating value. The optimal number is based on the local climate.

Solar Heat Gain Coefficient (SHGC): Measures how well a window unit blocks heat from incoming sunlight. These numbers range from 0 to 1. The lower the number, the less solar heat gained by the house. The optimal number is based on the local climate.

Visible Transmittance: The measurement of how much visible light comes through the window unit. These numbers also range from 0 to 1. The higher the number, the more light is passed through the window.

Air Leakage: The amount of air leakage through a square foot of window area, measured in cubic feet. The lower the number, the less air is passed through the window unit. These numbers usually range between 0.1 and 0.3.


Considerations for Homeowners

There are many ways to increase the thermal resistance of your windows. In recent years, the development of using low-E coatings has had a great effect on window energy performances. A low-E coating is a microscopic layer of transparent metal applied to one of the glass surfaces in the sealed space of an insulating glass unit (IGU). In a standard IGU, with no coating, the majority of heat transfer across the gap is with thermal radiation. Alternatively, a low-E coating blocks most of this heat loss. In fact, a low-E coated, double-paned window insulates as well as an uncoated quad-pane glass window. With the low-E coating’s reduction in heat loss, the inside glass surface temperature is warmer in cold weather and cooler in warm weather. When comparing a low-E IGU to a standard IGU, the U-Factor is reduced 50%, SHGC reduced 50%, and Visible Transmittance reduced 10%. These reductions can dramatically help reduce heat loss and create a more comfortable indoor environment.